1. Field Of The Invention
The present invention generally relates to apparatus and methods for fabricating integrated circuits and, in particular, to an apparatus and method for fabricating a curved grating in a broad area surface emitting distributed feedback semiconductor laser diode device.
2. Description Of The Prior Art
Since the early 1970's, surface emitting laser diode devices having a variety of different waveguide grating structures have been explored. Of those explored, distributed Bragg reflector (hereinafter DBR) laser diode devices and distributed feedback (hereinafter DFB) laser diode devices have attracted the most interest. DBR laser diode devices have gratings at the ends of an electrically pumped region to provide a feedback of photon radiation back into the pumped region. DFB laser diode devices have a continuous grating along one side of an electrically pumped region to provide a feedback of photon radiation back into the pumped region. To date, mainly DBR devices have been demonstrated. However, due to a high thermal resistance from a substrate side heat sink bonding configuration, these devices have had to be operated in a pulsed mode. Furthermore, when manufacturing a DBR device, precision mechanical cleaving of the end gratings is required. More recently, DFB devices have been demonstrated. These devices offer the promise of reliable high-power continuous wave operation due to an epitaxial side heat sink bonding configuration. Furthermore, surface emitting DFB laser diode devices generally require only conventional integrated circuit fabrication techniques during their manufacture.
To date, surface emitting distributed feedback semiconductor laser diode devices have been fabricated with a second order linear grating etched into a cladding layer surface of a semiconductor wafer. This type of grating can be fabricated by creating an optical standing wave on a photoresist coated wafer surface using two-beam interference. The wafer is then subjected to an ion milling process and a chemical etching process to transfer the photoresist exposed grating pattern into the cladding layer surface of the wafer.
A surface emitting distributed feedback semiconductor laser diode device having a second order linear grating that is fabricated according to the above-described technique produces unidirectional, monochromatic, coherent visible light through stimulated emission in its semiconductor materials. Such a device has a positively doped side and a negatively doped side that are joined at a junction, and the second order linear grating is etched into an outer surface of the positively doped side. The surface of the grating, upon which a highly conductive material is disposed in order to create an electrically pumped region, provides a means by which coherent photon energy fields may be diffracted. The second order grating design permits deflections of coherent photon radiation to be directed normal to an output window etched into the negatively doped side of the junction through first order diffraction, and directed parallel to the surface of the grating through second order diffraction. The first order diffraction produces a beam of unidirectional, monochromatic, coherent visible light at the output window, whereas the second order diffraction provides a feedback of photon radiation to the electrically pumped region that is adjacent and parallel to the surface of the grating. Much has been 7ritten on the subject of surface emitting distributed feedback semiconductor lasers in recent years and some good descriptive background articles on these devices are Surface Emitting Distributed Feedback Semiconductor Laser, Applied Physics Letters, Volume 51, Number 7, pp. 472-474, August 1987, and Analysis of Grating Surface Emitting Lasers, IEEE Journal of Quantum Electronics, Volume 26, Number 3, pp. 456-466, March 1990.
Along the length of a surface emitting distributed feedback semiconductor laser diode device having a second order linear grating, a longitudinal mode near-field output intensity profile is double-lobed and antisymmetric with a zero intensity null at the output window center. A corresponding longitudinal mode far-field output intensity profile is double-lobed and symmetric about the output window center. These longitudinal mode output intensity profiles have been substantiated in actual device measurements, although in these measurements it has been found that spontaneous emission partially fills the near-field intensity null at the output window center. Nonetheless, the longitudinal mode output intensity profiles associated with a surface emitting distributed feedback semiconductor laser diode device having a second order linear grating are acceptable for many applications due to a consistent mode relationship between the first and the second /rder diffracted photon radiation along the length of the electrically pumped region.
Along the width of a surface emitting distributed feedback semiconductor laser diode device having a second order linear grating, however, a lateral mode near-field output intensity profile and a lateral mode far-field output intensity profile are not acceptable for many applications due essentially to self-guiding and filamentation effects. Such effects result in the lateral mode near-field output intensity profile having an increasing number of lobes as the width of the electrically pumped region increases. Furthermore, there is a 180.degree. near-field phase shift between each near-field lobe. The corresponding lateral mode far-field output intensity profile displays a double-lobed pattern centered about the location of each 180.degree. near-field phase shift.
The reason that the lateral mode output intensity profiles associated with a surface emitting distributed feedback semiconductor laser diode device having a second order linear grating are not acceptable for many applications is that it is often required that the width of the electrically pumped region be increased, or a broad area electrically pumped region be created, in order to increase the output beam power of the device. Such an increase in the width of the electrically pumped region, although allowing an increase in the power applied to the device, will result in an increase in the number of lateral mode far-field lobes, which is unacceptable for many applications. It is therefore desirable to create a broad area surface emitting distributed feedback semiconductor laser diode device that provides an increase in output beam power, while concentrating that output beam power into a single lateral mode far-field lobe.